PROJECT TITLE Sources of Cholinergic Modulation of Cortical Microcircuits PROJECT SUMMARY The neurotransmitter acetylcholine (ACh) is important for attention and implicated in the cognitive symptoms of schizophrenia. In the cerebral cortex, rapid release of ACh acts as an attention signal, activating a disinhibitory circuit that ?frees? projection neurons to receive input from the thalamus and transmit information to other brain areas. Our lab uses the primary somatosensory cortex (S1) of mice to model basic, highly conserved aspects of cortical circuitry. In a recent paper, we showed that endogenously released ACh strengthens intra-cortical excitation in a cell type- and synapse-specific way in mouse S1. We also find intrinsic responses to ACh in different cell types, mediated distinctly by nicotinic and muscarinic receptors. To fully understand how ACh modulates circuits underlying cognition, we need to know the sources of ACh modulation of cortical excitability. The main source of cortical ACh is the basal forebrain. However, there are also cortical ACh neurons of unknown function which could potentially influence cholinergic signaling in a sparse, targeted manner. In Aim 1, we propose to anatomically assess the relative contribution of basal forebrain and cortically-derived ACh to cholinergic innervation of four neuron types, and functionally assess the contribution of these inputs to postsynaptic ACh responses using whole-cell recordings with optogenetics in brain slices. We hypothesize that cortical cholinergic cells get basal forebrain inputs and selectively target interneurons in deeper cortical layers, in contrast to the spatially diffuse, cell type-nonspecific inputs from the basal forebrain. We suspect that ACh selectively mediates thalamocortical excitation as well. In a recent paper, we found that endogenous Ach strengthens intra-cortical excitatory synapses onto somatostatin, but not PV, interneurons. However, activating cholinergic receptors pharmacologically strengthens both types of synapses, suggesting that physiologic ACh release is spatially segregated to target certain synapses. Prior pharmacological work suggests that thalamic inputs from the ventral posterior medial nucleus (VPM, a lower-order sensory thalamic nucleus) express nicotinic receptors, but it is unclear whether these inputs are strengthened by endogenous ACh, or whether inputs from a higher-order thalamic nucleus (the posterior medial nucleus, POm) could also be facilitated. Thalamic inputs show plasticity after attention-based learning, and we suspect that ACh may shift cortical circuits to favor different thalamic inputs during states of attention. In Aim 2, we will anatomically assess whether ACh release sites are located on VPM and POm thalamic inputs, and functionally assess whether endogenous ACh strengthens VPM and POm inputs using whole-cell recordings with dual-color optogenetics in brain slices. We hypothesize that ACh release sites are more commonly associated with POm inputs (to layers 2 and 5) than VPM inputs (to layer 4), and that endogenous ACh release has time-variable, facilitating effects on POm and VPM synapses.